Why is Sefer Bamidbar called 'Chomesh ha'Pekudim'?
Rashi (in Sotah, 36b): Because it begins with the appointment of the princes of the tribes. 1
And 'Pekudah' can mean 'appointment'.
What is the connection between the current Parshah to the previous one?
Ramban (in his Introduction to Sefer Bamidbar): Notwithstanding the break to present the Dinim of Shemitah and Yovel, 1 having discussed the Korbanos in Sefer Vayikra, the Torah now discusses the Ohel Mo'ed 2 - how they guarded it and its Keilim, how they encamped around it and how the people kept their distance, how the Kohanim behaved when they were encamped, when they carried it and whilst guarding it.
What sets apart the Mitzvos that are said in Sefer Bamidbar from those that were said previously?
Ramban: Or to the miracles that they experienced - in order to relate the wondrous deeds that Hashem performed with them (It is unclear why the Ramban does not include the incidents where they sinned and the punishments that they received?)
Ramban: With the exception of a few Mitzvos pertaining to the Korbanos the details of which they were not commanded earlier.
Why did Hashem order the counting of Yisrael here?
Rashi: Because Hashem was coming to rest His Shechinah on them, 1 one month after the Mishkan was erected.
Rashbam: Because they were about to enter Eretz Yisrael, 2 and begin the conquest of the land; and all those from the age of twenty (who were being counted) were eligible for conscription.
Seforno: To prepare them to enter Eretz Yisrael and to posses it without raising a sword. 3
He counted them when they left Egypt and after they fell at the Eigel. Why did He find it necessary to count them at every opportunity?
Rashi and Ramban (on Pasuk 45, citing Chazal): Is is a sign that Hashem loves Yisrael. 1
Why does the Torah see fit to mention the fact that this took place in the Ohel Mo'ed?
Ramban (in his introduction to Sefer Bamidbar): Having interrupted the sequence with the Mitzvos concerning Shemitah and Yovel, which were told to Moshe on Sinai, 1 it found it necessary to mention that, from now on, the Mitzvos will be said in the Ohel Mo'ed, like those that were said from the beginning of Vayikra until Parshas B'har.
Rashbam: In connection with all the commands that Hashem issued in the first year - before the Mishkan was erected, the Torah writes "Har Sinai"; 2 Once the Mishkan was erected, it writes "be'Midbar Sinai be'Ohel Mo'ed".
Rashbam: See, for example, Shemos 31:18. This is also borne out by the fact that, later, in 3:1. the Torah inserts the words "Ve'eileh Toldos Aharon u'Moshe ... be'Yom Diber es Moshe ... be'Har Sinai", which it adds because once the Mishkan was erected, Aharon's children no longer numbered four, but two, as the Torah stresses there, in Pasuk 4.
What is the difference between the Mitzvos that were said on Har Sinai and those that were said in the Ohel Mo'ed?
Ramban: There is no basic difference between them. The Torah confines the Shechinah to an enclosed space in the Mishkan (Shemos 19:12), just as it was on Har Sinai; 1 It commands that 'a Zar who approaches will die' (later, 1:51 & Bamidbar 18:7) just as it did on Har Sinai; 2 'Not to come and watch as the Kodesh is put away' (later, 4:20), just as they were commanded at Har Sinai 3 and that the Kohanim should guard the Mishkan and the Mizbe'ach (Bamidbar 18:5), just as they were at Har Sinai. 4